Copolymers of vinyl aromatic monomers such as styrene and esters of ethylenically unsaturated carboxylic acids such as the acrylate and methacrylate esters have been know for some time. These polymers are useful in a number of applications in which clarity is required. These polymers are used in a number of applications such as housewares including glasses and food containers. It is always desirable to improve on the toughness of these types of polymers. However, it is also desirable to improve on their environmental stress crack resistance. Environmental stress cracking is the development of small cracks in the plastic on exposure to substances such as detergents and fats and oils. Over time with repeated exposure to such substances the number of these cracks increase leading to an increased haze in clear products and ultimately the failure of the plastic. It would be desirable to improve the environmental stress crack resistance of such copolymers and to also improve the impact toughness of the alloys while maintaining excellent clarity.
U.S. patent application Ser. No. 654,058 in the name of W. G. Blasius, assigned to Polysar Financial Services S.A. (now, Novacor Chemicals (International) S.A.) discloses and claims alloys comprising from 30 to 82 weight % of a clear brittle polymer, such as a styrene acrylate, from 3 to 50 weight % of a rubbery polymer and from 15 to 67 weight % of a ductile polymer. The Blasius patent application contemplates lower amounts of the styrene acrylate polymer than contemplated in the present case. Furthermore, the Blasius application does not disclose that the products have an improved environmental stress crack resistance.
U.S. patent application Ser. No. 758,802, filed Sep. 12, 1991 in the names of D. Hauser, M. Colella and W. G. Blasius discloses and claims the use of the alloys of the type disclosed in the Blasius application in house wares. The utility of the patent application is that the housewares have better environmental stress crack resistance. The present patent application is limited to the case where there is a lower content of styrene acrylate. There is nothing in the Hauser patent application which discloses that compositions having a higher content of styrene acrylate would also have improved environmental stress crack resistance. In fact, as the Hauser application teaches, the improvement to environmental stress crack resistance is due to the components other than the styrene acrylate polymer, increasing this component would be expected to reduce the environmental stress crack resistance. PG,4
Chemical Abstracts 109:171278v [Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1988, 29(2), 180-1] discloses that physical blends of block SBR (styrene content: 75.2 weight %) and a copolymer of methyl methacrylate and styrene are partially compatible. The morphological and mechanical properties of the blend are strongly dependent on the molding/mixing process used to produce the blend. The SBR could only be toughened when the ideal morphology of the blend was obtained.
Chemical Abstracts 106:85901z [JP 61/200,151] discloses thermoplastic compositions comprising from 5 to 95 weight % of a block copolymer and from 95 to 5 weight % of a thermoplastic polymer (e.g. ABS). The block copolymer is derived from coupling an aromatic vinyl polymer block with an aromatic vinyl monomer-conjugated diene copolymer block. The subject thermoplastic compositions apparently have improved impact strength.
Chemical Abstracts 112:8285y [JP 61/291,610] discloses the use of "impact modifiers" for thermoplastic polymers. The modifiers are basically similar to the block copolymer disclosed by Chemical Abstracts 106:85901z.
Chemical Abstracts 99:196070j [JP 58/122,954] discloses improvement of anisotropy of a styrene hydrocarbon-conjugated diene block copolymer by blending it with a styrene-methacrylic ester copolymer. Such a blend, when extruded to form a molded plate, apparently possesses superior physical properties.
Chemical Abstracts 111:8522c [JP 01/45614] discloses high-impact styrene polymer compositions comprising a major amount of a styrene (50 to 90 weight %)-conjugated diene (50 to 10 weight %) block copolymer and a minor amount of a styrene-based polymer (e.g. styrene-methyl methacrylate copolymer).
The present invention seeks to provide a clear polymer having good environmental stress crack resistance. Additionally the polymer should have improved toughness.